Wardo

Wardo

Active Member
I want to put a cannon plug on the firewall for my engine monitoring wires for the skyview.

I talked to a tech guy at Dynon and told him that my concern was the alumel chromel wires that go to the CHT & EGT sending units. The dissimilar metal of the cannon plug pins could cause a false indication although the connectors from the wires to the sending units are not alumel chromel. He wasn't sure what to say about it, whether or not it would effect the reading.

Might not be a factor. has anybody done this with any of the "Brown" dynon temp sending wires.

Pics and input welcome.--
 
Feel like experimenting?

Theory says that the metal A (thermocouple wire) junction to metal B (pin) will be cancelled out by the Metal B (pin) to metal A (thermocouple wire) as you transition through the plug. Of course, the two junctions need to be at the same temperature to zero out. Would they be on a cannon plug on the firewall? I would think they would be very close indeed, but close enough??? Dunno.

See this link http://www.sterntech.com/pdfs/thermocouples.pdf

Perhaps a test is in order... a baseline thermocouple reading followed by the wire cut and splice with male/female pins ... lightly heated on one end.

Don
 
You bet there will be an instrument error if you do not use alumel chromel pins in the cannon plug. Silver solder is normally used to attach the terminal end or cannon plug pin to the wire. In my day CHT gauges used iron constantine wires. I suspect the EGT could also be iron constantine.
I had a Jet Ranger with crimped copper terminal ends on a TOT harness (alumel chromel). We chased high TOT problems with this aircraft until we finally stumbled across these incorrect terminals. Changing the TOT harness fixed the problem.

Ray
 
Alumel Chromel Pins

So, it seems pretty clear, get a cannon plug that has alumel chromel pins.

Any ideas on where to get these.
 
Wardo

I hesitate to wade in?. I?m a new builder and a long time site stalker.:eek: I have been working Avionics with the Air Force for 30 years on F-111s and F-15s. We use to pay a lot of attention to alumel chromel pins with silver solder. About 10 years ago the connectors became very hard to come by. Engineering decided it was not that critical at the connectors. So we started to use normal pins. We have not had a problem over the past 10 years. I do not know the tolerance of the equipment you are installing. Check with the manufacturer again and find out how critical the pins are.
 
My question is what benefit are you gaining by doing this? I hate to be a "Debbie Downer" because I put cannon plugs on the firewall in my first RV, but the reality is it adds little postiive value and the offset is negated by the potential future problems of having a plug.

In a pressurized airplane we have a reason to do this, in an RV not so much. Are you going to run the P-Leads through a cannon plug? How about the control cables, fuel lines, etc..? Remember that the entire firewall is held together with aluminum rivets. My point is that running the thermocouple wires through a cannon plug doesn't add a lot of value in an RV. Gremlins love to make circular connectors home!

The number one law in avionics is "The leading cause of problems is solutions", and "don't try to create a solution to a problem that doesn't exist".

That's just my 2 cents as usual.

Cheers,
Stein

BTW, Bnut is pretty much right (depending on the brand of system we're talkinga bout) regarding the use of standard pins over thermocouple pins.
 
Thru hole with fire block.

I looked for a long time for the correct Cannon connector to do this. After much debate on the cost (and time I was spending on the search), I decided to punt. In the time I wasted searching for the solution I could have removed and reinstalled the probes a dozen times. I really liked the idea but it's hard to justify in reality.
 
Stein,

I agree!!! The fewer breaks in a wire the better and pins/connectors are the weakest point. It really comes down to whether you will need to disconnect because of maintenance at some point. Then it would be handy. If not, don?t do it!! Just my $0.02 worth.
 
I must be weird because I've got an extreme bug about ease of disassembly. I've had so much trouble and gotten so gashed up pulling stuff out of airplanes that for me, one of my "satisfactions" is to make stuff come back apart easily. Any time I can make something come back apart quickly with no parts damage and then go back together again the same way, I get a little rush. As a result, I use lots of connectors, modular panel sections, etc. Takes way longer (the first time). Introduces more potential failure points, etc. I do it anyway. The type K TC's we use most of seem totally happy with circular connectors & gold pins. I've done a few engine TC harnesses this way (3 RV's). I've also checked gage readings through the pins vs a straight wire and they were so close I couldn't tell which was more right. If an engine has an oil blow or needs to come out, I like it to be QEC easy to strip & clean. A circular connector like this also gives you quick access to check continuity, resistance, voltage, etc. Of course, half the time the problem is in the plug itself. Plus, if you use a connector with extra pins, then you can add something later without another penetration or anything. So, good, bad, sometimes ugly.
 
I've wired up a few RVs now for friends, and the past two (RV-8 and RV-10) we used the combo stainless steel & aluminum "eyeballs" from Spruce. Bought the eyeballs with the largest center hole available, and drilled out the hole even larger... big enough to hold a large bundle of wires, encased in thick heatshrink as the bundle passed thru the eyeball. We used two such eyeballs on each RV, one for the thermal probe wires, and one for the rest of the engine compartment wires. You have to be absolutely certain, you have *all* the wires you'll every need before heatshrinking them together at the eyeball pass-thru, as you cannot add any more strands unless you re-do the whole bundle, but when it's all done, it looks great and provides a secure wire penetration thru the firewall. Don't worry about an engine fire melting thru the aluminum part of the eyeball at all, because an engine fire is going to melt thru your aluminum cabin heat duct/air mixer box first and that big hole thru your firewall right there, and kill you long before it'll melt thru such a wiring pass-thru / eyeball.
 
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SteinAir said:
My question is what benefit are you gaining by doing this? I hate to be a "Debbie Downer" because I put cannon plugs on the firewall in my first RV, but the reality is it adds little postiive value and the offset is negated by the potential future problems of having a plug.

In a pressurized airplane we have a reason to do this, in an RV not so much. Are you going to run the P-Leads through a cannon plug? How about the control cables, fuel lines, etc..? Remember that the entire firewall is held together with aluminum rivets. My point is that running the thermocouple wires through a cannon plug doesn't add a lot of value in an RV. Gremlins love to make circular connectors home!

The number one law in avionics is "The leading cause of problems is solutions", and "don't try to create a solution to a problem that doesn't exist".
Click to expand...

Stein nailed it, pretty much a waste of time and $$$ IMO, and many more potential failure points to boot.

And don't forget the mi-spec ($$$) crimpers to do the pins with, it makes no sense to buy $800 in fancy connectors then use your $29 crimper to put them together.

A QEC (quick enging change) Lyc, maybe the Reno boys do this but makes little practical sense. It would look cool though.
 
Cannon plug

Thanks all for your input, keep it coming. Throughout my build I am trying to incorporate solutions for all the complaints that i have with aviation maintenance. For several years now I've made my living in aviation maintenance, maintaining everything from Corsairs to J-3s. As a business owner you try to do things as efficiently (quickly) as possible to keep the cost down while still maintaining a very high level of quality.

For example: last weekend I began the condition inspections on two RVs, -6 -8, at my home drone, Sky Ranch, and ran into this particular problem when I found cracks on the gear/engine mount on the RV-6. The engine and engine mount had to be removed from the aircraft to facilitate repair. If there would have been a cannon plug on the firewall for just half of the wires that I had to disco from the engine, things would have gone much faster-cheaper.

As the primary mechanic and resident on a private flying community type airport, I try to help my buds out where I can and things like a cannon plug or two on the firewall of some of these planes would make the difference in situations like this. Granted, you're going to have to disco some things from the firewall-engine, I get it.

As far as there being problems with cannon plugs, I will not argue with the experts, Stein, but I will say that I have installed many MIL-SPEC cannon plugs while employed by a DoD contracted mod facility and cannot speak of to many issues that weren't installer error, which is true in most facets of aviation, human error.

The crimp tools are not the problem, Ive got plenty of DMC crimpers and dies, its finding a source for the plugs that are compatible with the alumel chromel temp sending wires.

I like things that are efficient, simple, low maintenance and effective. A few of the things that I'm employing in my build for those reasons are a 30A, vacuum pump pad mounted primary alternator, P-MAGS, 90 deg oil filter adapter, cannon plugs for empennage, both wings and hopefully the firewall, Michelin Air-Stop tubes, Grove gun drilled gear, LED lights throughout, Odyssey battery, SEAL-ALL plugs on most of the internal electrical components like a/p and trim servos- flap motor-fuel pump- and TCW products......
 
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